Adjustable screen printing chase

ABSTRACT

A laterally shiftable screen supporting bar within a U-shaped channel member of a screen printing frame assembly is provided with an arcuate slot extending inwardly from the surface of the bar which faces the channel opening and receives a similarly shaped arcuate screen clamping strip with the screen edge held therebetween such that the force acting through the screen on the strip tending to move it linearly out of the arcuate slot is resisted by the arcuate interlock formed thereby. A projection on the strip engages a flange on the channel member to prevent arcuate removal of the strip from the channel member slot.

O United States Patent 1 in] 3,788,216 Lambert Jan. 29, 1974 ADJUSTABLE SCREEN PRINTING CHASE Primary Examiner-Robert E. Pulfrey [75] Inventor. Daryl G. Lambert, Phoenix, Ariz. Assistant Examiner RI E. suter [73] Assignee: The Dia-Print Company, Inc., Attorney, Agent, or F irm-Sughrue, Rothwell, Mion,

PhOefliX, AriZ- Zinn & Macpeak 22 F1 m red Dec 8, 1971 ABSTRACT [21] Appl' 205944 A laterally shiftable screen supporting bar within a U- shaped channel member of a screen printing frame as- [52] US. Cl 101/ 127.1, 10l/4l5.l, 160/395 sembly is provided with an arcuate slot extending in- [51] Int. Cl B4ln 1/24 wardly from the surface of the bar which faces the [58] Field of Search 101/ 127. 1, 415.1; 209/405, channel opening and receives a similarly shaped arcu- 209/406, 407, 408; 51/370 ate screen clamping strip with the screen edge held therebetween such that the force acting through the [56] References Cited screen on the strip tending to move it linearly out of UNITED STATES PATENTS the arcuate slot is resisted by the arcuate interlock 2,894,455 7/1959 Lambert 101/127.1 x formed thereby A Prolectio on the Strip engages a 3,126,332 3/1964 Salem flange on the channel member to prevent arcuate re- 3,230,872 1/1966 DeGroot moval of the strip from the channel member slot. 3,608,482 9/1971 Bubley l0l/l27.l 9 Claims, 6 Drawing Figures PATENTEU JAN 2 9 i974 PRIOR ART ADJUSTABLE SCREEN PRINTING CHASE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a chase or frame assembly for screen printing and, more particularly, to a simplified yet effective locking arrangement for locking the marginal edges of the screen to the clamping bars carried within U-shaped channel members defining the rectangular screen printing frame.

2. Description of the Prior Art Fine metallic screens are highly useful in screen printing, especially in the production of printed electrical circuits." While the texture of the fine metallic screen material has superior resistance to wear and indifference to atmospheric conditions, such screens are required to be set or held within frame assemblies which permit variable screen tension to conform the screen to the exacting requirements of high tolerance printing. One type of adjustable screen printing frame assembly which has been found highly useful in the production of printed circuits for electrical use is illustrated in US. Pat. No. 2,894,455 issuing July 14, 1959, and entitled Adjustable Screen Printing Frame.

The frame assembly of that patent comprises a rectangular main frame composed of four channel-defining members, joined together to present a substantially smooth inner wall, with one edge of the wall having a rounded peripheral portion to present substantially line contact to the printing screen stretched across the same. The screen is coupled to laterally shiftable screen supporting bars or holding members, one for each wall of the frame, and positioned within the channel defining members. Each channel defining member includes a narrow recess at the bottom thereof and at right angles to the major channel opening and the bar is generally L-shaped in configuration formed of a first flange which carries the edge of the screen locked thereto and a second thin flange at right angles thereto which is received within the narrow recess of the channel defining frame member. In this manner, the thin flange, being closely received by the recess allows lateral shifting of each bar with respect to the longitudinal axis of the channel defining member but prevents rotation of the bar during such lateral movement and tensioning of the screen.

Within the same patent, the manner of attaching the edges of the screen to respective screen-holding bars consists of providing within the surface of the bar facing in the same direction as the channel opening, an L- shaped recess which receives oppositely directed mated strips of L-shaped cross sectional configuration sandwiching the screen therebetween. The configuration of the two oppositely oriented L-shaped mated strips and the portion of the screen edge captured therebetween substantially fills the recess and frictionally locks the edge of the screen within the transversely or laterally shiftable bar.

While this-provided a highly satisfactory assembly, especially for precision screen printing of electrical printed circuits and the like, the coupling of the screen marginal edges to the respective bars requires the employment of two cooperating strips while, further, the pulling force exerted on the locking strip by the tension on the screen is merely opposite to the direction of insertion of the same. This reduces the reliability in the creation and maintenance of the necessary tension in the screen. In clamping, the screen frame in such environment must provide four complementary functions: first, the chase or frame must be capable of unkinking" the steel or other metallic mesh defining the screen. When the mesh is woven, one filament, the warp is wrapped around the other filament, the weft. When the mesh is pulled taut, it should equalize the curvature for both the warp and the weft. Thus, it must create a nearly identical norm for both filaments. If this is not done, skidding of the image will result in the direction of the improperly stretched filament. Secondly, the chase or screen frame must equalize the filament tension. Due to the tolerance of the filaments during drawing, plus the variation in tensile strength of each filament, the locking device must, of necessity, accommodate these discrepancies. Failure to do this will result in some filaments, those which are oversized and harder than their weaker neighbors, being broken. This may play havoc with dimensions and can contribute to poor quality of line reproduction. Further, such unequalization is very difficult to ascertain without the benefit of magnification. Assuming un-kinking is achieved, the next function of the locking device and the structural integrity of the outer channel of the frame must be such as to purposely elongate the steel filament short of the breaking point, since failure to accomplish this will result in bagging of the mesh during multiple imprinting cycles. Finally, the proper tautness must approach that of a solid sheet of steel with the maximum rigidity required not inhibiting the ability of the frame to print off contact and yet it must achieve this without distorting the image within four place decimals.

While the wedge lock" principle of clamping the screen to the laterally shiftable bar as set forth in my prior patent adequately performs the four functions outlined above, assembly necessitates the placement of two oppositely directed but identical, L-shaped locking strips within a specially formed L-shaped recess on the outwardly facing edge of the clamping.

SUMMARY OF THE INVENTION The present invention is directed to an improved adjustable frame assembly for screen printing of the type in which a rectangular main frame constituting multiple joined channel defining members carries a like number of screen holding bars corresponding to the channel defining members and positioned within the channels carried thereby. Means couple the holding bars to the channel defining members to effect adjustable shifting of the bars laterally within the channels relative to the longitudinal axis of the channels to stretch the screen. The screen, in turn, is carried by coupling the marginal edges of the screen to respective bars.- The improvement constitutes an arcuate, elongated slot extending inwardly from the face of each bar facing outwardly of the channel and a corresponding arcuate locking strip positioned within each slot with the marginal edge of the screen wrapped thereabout. Thus, tension on the screen exerts a linear force tending to pull the arcuate locking strip linearly out of the arcuate slot but is resisted by the arcuate interlock defined by these members.

In one form, the channel defining frame members, each include a narrow recess at right angles to the main channel opening and the bar includes a first flange carrying the arcuate slot and a thin flange extending at right angles thereto which, in turn, is received within the frame member recess to prevent rotation of the bar during lateral movement within the channel and tensioning of the screen. The arcuate slot carried by each clamping bar lies preferably intermediate the sides of the channel carrying the flange and each channel defining member includes a recess within the outer sidewall which faces the channel opening. The locking strip includes a right angle edge projection received within this sidewall recess to further locate the strip relative to the arcuate slot. The edge projection of the arcuate strip is preferably provided with a convex face directed toward the channel bottom and the edge of the bar, across which the strip projection extends is concave to conform to the irregular convex face of the edge projection to further assist locking the strip within the arcuate slot receiving the same.

In a second form, the channel defining frame members lack the narrow recess and the bar is devoid of a thin flange. The arcuate slot is generally half moon in cross sectional configuration and faces the open end of the channel. A projection of the bar parallel to the plane of the screen extends toward the edge of the screen to partially overlie the slot. The locking strip is spoon shaped in cross section with a base correspondingly half moon in cross section rotatably inserted within the half moon shaped slot, with the lip underlying the bar projection. A handle shaped portion of the locking strip is received within a channel defining member sidewall recess to prevent rotative withdrawal of the locking strip upon lateral movement of the bar within the channel in a direction stretching the screen.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a bottom plan view of an assembled screen printing frame of the present invention.

FIG. 2 is a sectional view of a portion of a prior art frame assembly.

FIG. 3 is a sectional view of the frame assembly of FIG. 1 taken about lines 33.

FIG. 4 is a sectional view of the assembly of the present invention illustrating the parts thereof in the process of assembly.

FIG. 5 is a sectional view of a portion of an alternative assembly of the present invention, in the process of assembly.

FIG. 6 is a similar section view to that of FIG. 5, with the screen fully stretched.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, the screen printing frame assembly of the present invention is illustrated generally at 10 and, while any regular polygonal arrangement is suitable, the most obvious useful configuration is one which is rectangular in shape. In this case, as illustrated, the frame assembly 10 is defined by four channel members 12, being generally formed of metal and in particular by extrusion so as to have the desired configuration. The extruded frame members are cut to effect the rectangular assembly of particular dimensions. As illustrated at 14, the members are fixedly coupled together by welding the joints between the same. A screen 16 is suitably supported by four independent clamping bars 18, each bar being located within a respective channel member 12. Preferably, the clamping bars are also formed as metallic extrusions. As further illustrated at 62, a plurality of Allen head adjusting screws maintain the clamping bars 18 within the channel members 12 and facilitate lateral adjustment of the bars with respect to the U-shaped channel members 12 to thereby effect proper tensioning to the metallic screen 16. Three Allen headed screws 62 are provided for each channel member 12, although more or less individual adjusting screws 62 may be employed if desired, at longitudinally spaced positions.

Prior to discussing the frame assembly of the present invention in detail, attention is directed to my prior screen printing frame assembly as sectionally illustrated in FIG. 2 which is set forth in my referred to US. Pat. No. 2,894,455. Similar elements to those employed in FIGS. 1, 3 and 4 carry like prime numerals. In similar fashion, this frame assembly 10 involved an outer channel defining member 12, which carry within the channels 42' clamping bar 18' to which the marginal edges of the screen 16' is directly attached. Briefly, the inner flange 20' of the channel member 12' terminates in a bulbous edge 26' which is recessed slightly from the terminal edge 28 of the outer flange 22 and web 24 defines each U-shaped channel 12'. This allows the frame assembly 10' to be placed upon a plane surface with the screen 16' clear of that surface so that there will be no contact between the screen and the surface. The bulbulous shape of the rim 26' prevents undue wear at this point on the screen and effects preferably line contact between the screen and the channel member 12. Outer flange 22' is generally triangular in cross section and a narrow recess 32 is formed within the same, adjacent web 24 to receive side flange 46' of an L-shaped or angle cross section clamping bar 18, the side flange 46 being joined directly to the somewhat thicker body flange 44 and the outer edge of the same being curved as at 48 at the area of junction between the two flanges. The triangular shaped outer flange 22' of the U-shaped channel frame member 12 is grooved on its outer face as at 64 and is drilled at longitudinally spaced positions to receive the Allen headed adjusting screws 62, the threaded ends of these screws being received within tapped and threaded holes 63' carried by the body flange 44'. This readily allowed the edge of the clamping bars to be shifted laterally of the channel longitudinal axis, that is, in a plane parallel to the plane of the stretched screen 16. The length of the side flange 46 is such that it cannot be inserted directly into the narrow recess 32 which receives the same but must be rotated during insertion. After insei'tion in this manner, it is obvious that taking up of the adjustment screws 62 results in lateral shifting of the bar relative to the channel axis rotational movement of the same being limited due to close fitting of the side flange 46' to the recess 32.

In the prior art arrangement of FIG. 2, to secure the screen, the marginal edges of the screen to respective bars 18, each of the bars 18' is provided with an L- shaped channel or slot 54 having a base recessed portion 58' directed toward the frame interior. Further, the edge 59' or the face of the bar directed away from the channel opening is offset on one side of channel 54 as at 61. This configuration allows clamping by cooperating pairs of beaded strips 56a and 56b which secure the screen after it has been cut to size by carrying the marginal portion of the same pressed within the slot 54. The bead on the strip 56a is sufficiently smaller than the base recess 58 to permit the bead plus the screen 16' to fit within the recess 58'. In the arrangement of my patent, twice the thickness of each beaded strip plus the thickness of the screen firmly fills the slot 54' with the strips 56a and 56b being of identical configuration. The strip 56a is first inserted in the slot 54' with the screen 16 to insure secure initial tensioning of the screen. Additional tension is obtained by holding the outer edge of the strip 56a to prevent its withdrawal from the slot while grasping the free edge of the screen and pulling it outwardly to cause the bit of the screen in the slot 54 to conform closely to the outer surface of strip 56a. With tension being exerted, the strip 56b is pushed in behind strip 560' in slot 54 to substantially fill the slot.

Suitable tension is then applied to each of the bars and thus the screen 16 by independently adjusting each of the Allen head screws 62.

Turning next to FIGS. 3 and 4, the improved screen printing frame assembly of the present invention is further illustrated in addition to FIG. I in which in similar fashion to my prior art arrangement of FIG. 2, similar elements will be given similar unprimed numerical designations. In this respect, each of the channel defining members 12 has extending inwardly from the upper edge 28 of the outer flange 22, a right angle projecting portion 29 which in conjunction with the sharply inclined outermost face 34 of the outer flange 22 defines a triangular shaped recess 31. The channel member 12 is similar to that of my prior patent with the exception that the inner flange is not provided with a bulbulous or rounded outer edge, the edge 26 in this case being flat and having the screen 16 stretched thereacross.

Turning to each of the clamping bars 18, it is noted that the clamping bar while being formed generally of a side flange 46 and a much thicker body flange 44, the body flange 44 in this case has an arcuate recess or slot 66 extending inwardly from the outer face 59, the sidewalls 68 and 70 of the same being concave and convex respectively and having essentially the same radius of curvature. The bottom wall 72 of the arcuate recess or slot 66 is flat and generally parallel to the outer face or surface 59. In similar manner to the illustration of FIG. 2, the outer surface 59 has a portion at the end of curved sidewall 68 offset at 61 terminating in an edge portion 63 which is concave in configuration.

A principal component of the clamping device of the present invention resides in a specially configured single element locking strip 74 consisting of an arcuate strip portion 76 havng integrally formed therewith a projection 78 extending generally at right angles thereto. The strip portion 76 is of generally the same arcuate configuration as that of slot 66. The projection 78 is provided with a curved surface portion 80 corresponding to the curved surface edge 63 of the clamping bar 18, and an adjacent inclined surface portion 82 which conforms generally to inclined surface 34 partially defining the triangular recess 31. Further, the outer face 84 of the arcuate strip projection 78 lies parallel to and inside the right angle projecting flange 29 when the clamping bar 18 is laterally moved to its outermost position within the channel shaped member 12 via screws 62. The screen itself is essentially wrapped about the locking strip 74 to frictionally maintain the respective marginal edges of the screen 16 coupled to the lateral adjustable clamping bar. In this respect, the force acting on the strip 74 tends to move the strip 74 linearly out of the slot 66. However, both the strip portion 76 and the slot 66 receiving the same are arcuate and can only move by rotation relative to each other. Since rotation is not permitted due to the abutment of strip projection 78 against flange projection 29, excellent locking of the marginal edges of the screen 16 to the clamping bar occurs.

Since all three members, that is, the U-shaped channel member 12, the clamping bar 18 and the locking strip 74 may be readily made of metallic extrusions, the frame may be made in a wide variety of sizes from standard stock. However, the frame member may be fitted together by mitering of the comers and welding as described previously, and the clamping bars need only to be cut to appropriate length along with the locking strip 74 to a similar length since both fit within the channels of the frame members 12.

An alternate form of the improved screen printing frame assembly of the present invention is illustrated in FIGS. 5 and 6 operating upon principles common with that of the first embodiment. Each of the channel members 112 in frame assembly 1 10 has extending inwardly from the upper edge 128 of the outer flange 122, a right angle projecting portion 129 which in conjunction with the sharply inclined face 134 of the side wall or outer flange 122, defines a triangular shaped recess 131. The channel member 112 is devoid of the narrow recess such as recess 32 of the prior art arrangement of FIG. 2, and in similar fashion to the embodiment of FIGS. 3 and 4, is not provided with a bulbous or rounded outer edge, in this case edge 126, over which the screen 116 rides, is flat, with the screen 116 being stretched thereacross.

Each of the clamping bars 118 is devoid of a side flange such as side flange 46 of the FIG. 3 embodiment, clamping bar 118 having rounded inner edges conforming to the curvature found at the bottom of the channel defined by channel member 112. The thick body of the clamping bar 118 is provided with a recess 166 which extends inwardly from the outer face 159 of the clamping bar, the clamping bar includes an integral projection 192 which partially overlies recess 166. The concave sidewall 168 of the clamping bar forms in conjunction with projection 192, a recess which is generally half moon in sectional configuration.

The third element of the assembly resides again in a specially configured, single element locking strip 174 which, in cross section, is generally spoon shaped including a base portion 176 which is half moon in cross section corresponding to recess 166, being provided with a lip 194 which fits just behind projection 192 when the locking strip is rotated into proper position with the edge of screen 116 being sandwiched between the locking strip and the clamping bar 118. Further, the locking strip is provided with a handle or projection 178 which extends outwardly from the base portion 176, and when in proper position, parallel to the axis of the adjusting screws 62, the outer face 184 of the projection or handle 178 lies parallel to and just inside the right angle projecting flange 129 of channel member 110. The handle 178 is received within the recess 131 when the clamping bar 1 18 is laterally moved relative to the channel axis, to its outermost position by rotating the adjustment screws 62. The screen 116 is essentially wrapped about the locking strip 174 to frictionally maintain the respective marginal edges of the screen 116 coupled to the lateral adjustable clamping bars. The handle or projection 178 is further provided with an inclined edge 196 which mates with the inclined surface or face 134 of flange 120 to prevent rotation of the locking strip 174 either clockwise or counterclockwise when the screen 116 is fully tensioned, as illustrated in FIG. 6. In similar manner to the prior embodiment, the force acting on the locking strip 174 tends to move the strip 174 linearly out of the arcuate or curved slot 166. However, due to the sizing of the engaged parts, and the fact that both the base portion 176 and the slot 166 receiving the same have arcuate or curved mating surfaces, these elements can move only by rotation relative to each other. Since rotation is prevented due to the abutment of the locking strip projection 178 against the flange projection 129 and the inclined surface 134, excellent locking of the marginal edges of screen 116 to the clamping bar occurs.

While each screen frame assembly has been described in conjunction with a metallic screen, especially useful in the screen printing of electrical printed circuits, it is obvious that the adjustable frame assembly may use various types of screens in the applied printing fields other than the manufacture of electrical printed circuits. Assembly is facilitated, maintenance of tension insured, and the time in both assembly and disassembly of the screen to the frame greatly reduced.

What is claimed is:

1. In an adjustable frame assembly for screen printing including a screen, a rectangular main frame constituting multiple edge joined channel members defining channels opening to one side of said frame, screen clamping bar members corresponding to said channel members positioned within the channels thereof, means for adjustably shifting the bar members laterally of the longitudinal channel axis, within said channels between a first screen unstretched position and a second screen stretched position and means for attaching the marginal edges of said screens to respective bar members, the improvement wherein said attaching means comprises:

a curved, elongated slot extending inwardly from the face contacting said screen of each bar member, a locking strip having a correspondingly curved interlocking portion positioned within each slot with a marginal edge of said screen wrapped about said locking strip and captured between said strip and said slot, and

abutment means carried by said locking strip projecting from said interlocking portion and engageable with a channel wall of the corresponding channel member for preventing rotational release of said strip from said slot during lateral shifting of said bar members relative to the longitudinal axis of said channels.

2. The adjustable frame assembly as claimed in claim 1, wherein said abutment means comprises an integral projection of said locking strip extending toward the outer edge of said frame and said channel wall is recessed to receive said strip.

3. The adjustable frame assembly as claimed in claim 2, wherein said projection of said locking strip extends enerally at right angles to a base portion thereof and includes a convex surface portion directed towards said channel and the edge of said bar adjacent said slot is of concave configuration conforming to the convex surface portion of the projection for further locking of the screen marginal edge relative to said bar.

4. The adjustable frame assembly as claimed in claim 1, wherein each channel is generally L-shaped in cross section including a narrow recess at its inner end, said clamping bar includes a first flange carrying said arcuate slot and a second, thin flange at right angles thereto, said second thin flange being carried by said narrow recess to prevent rotation of the bar during lateral movement of said bar relative to the channel axis within the channel during tensioning of said screen.

5. The adjustable frame assembly as claimed in claim 2, wherein each channel is generally L-shaped in cross section including a narrow recess at its inner end, said clamping bar includes a first flange carrying said arcuate slot and a second, thin flange at right angles thereto, said second thin flange being carried by said narrow recess to prevent rotation of the bar during lateral movement of said bar relative to the channel axis within the channel during tensioning of said screen.

6. The adjustable frame assembly as claimed in claim 3, wherein each channel is generally L-shaped in cross section including a narrow recess at its inner end, said clamping bar includes a first flange carrying said arcuate slot and a second, thin flange at right angles thereto, said second, thin flange being carried by said recess to prevent rotation of the bar during lateral movement of said bar relative to the channel axis within the channel during tensioning of said screen.

7. The adjustable frame assembly as claimed in claim 1, wherein said curved elongated slot is generally half moon in cross sectional configuration, said bar includes a projection partially overlying said slot and running the length of said slot, said locking strip is spoon shaped in cross sectional configuration including a base portion of similar half moon cross sectional configuration to said slot and having a lip underlying said clamping bar projection, and said locking strip further includes a handle like projection in engagement with said channel member to prevent rotational release of the locking strip from the slot during lateral shifting of the bar members relative to the longitudinal axis of said channel members.

8. The adjustable frame assembly as claimed in claim 7, wherein the channel wall remote from said screen is recessed longitudinally and the locking strip handle like projection is received by said recess.

9. The adjustable frame assembly as claimed in claim 8, wherein said recess within said channel wall is inclined, the handle like projection of said locking strip carries an inclined edge which mates with the inclined surface of said recess when said bar is in said second position and said locking strip is prevented from rotating in a first direction by said lip contacting said bar member projection and said locking strip projection contacting one wall of said channel wall recess, and said locking strip is prevented from rotating in the opposite direction by said locking strip projection, contacting the other wall of said channel wall recess.

* i l II! 

1. In an adjustable frame assembly for screen printing including a screen, a rectangular main fRame constituting multiple edge joined channel members defining channels opening to one side of said frame, screen clamping bar members corresponding to said channel members positioned within the channels thereof, means for adjustably shifting the bar members laterally of the longitudinal channel axis, within said channels between a first screen unstretched position and a second screen stretched position and means for attaching the marginal edges of said screens to respective bar members, the improvement wherein said attaching means comprises: a curved, elongated slot extending inwardly from the face contacting said screen of each bar member, a locking strip having a correspondingly curved interlocking portion positioned within each slot with a marginal edge of said screen wrapped about said locking strip and captured between said strip and said slot, and abutment means carried by said locking strip projecting from said interlocking portion and engageable with a channel wall of the corresponding channel member for preventing rotational release of said strip from said slot during lateral shifting of said bar members relative to the longitudinal axis of said channels.
 2. The adjustable frame assembly as claimed in claim 1, wherein said abutment means comprises an integral projection of said locking strip extending toward the outer edge of said frame and said channel wall is recessed to receive said strip.
 3. The adjustable frame assembly as claimed in claim 2, wherein said projection of said locking strip extends enerally at right angles to a base portion thereof and includes a convex surface portion directed towards said channel and the edge of said bar adjacent said slot is of concave configuration conforming to the convex surface portion of the projection for further locking of the screen marginal edge relative to said bar.
 4. The adjustable frame assembly as claimed in claim 1, wherein each channel is generally L-shaped in cross section including a narrow recess at its inner end, said clamping bar includes a first flange carrying said arcuate slot and a second, thin flange at right angles thereto, said second thin flange being carried by said narrow recess to prevent rotation of the bar during lateral movement of said bar relative to the channel axis within the channel during tensioning of said screen.
 5. The adjustable frame assembly as claimed in claim 2, wherein each channel is generally L-shaped in cross section including a narrow recess at its inner end, said clamping bar includes a first flange carrying said arcuate slot and a second, thin flange at right angles thereto, said second thin flange being carried by said narrow recess to prevent rotation of the bar during lateral movement of said bar relative to the channel axis within the channel during tensioning of said screen.
 6. The adjustable frame assembly as claimed in claim 3, wherein each channel is generally L-shaped in cross section including a narrow recess at its inner end, said clamping bar includes a first flange carrying said arcuate slot and a second, thin flange at right angles thereto, said second, thin flange being carried by said recess to prevent rotation of the bar during lateral movement of said bar relative to the channel axis within the channel during tensioning of said screen.
 7. The adjustable frame assembly as claimed in claim 1, wherein said curved elongated slot is generally half moon in cross sectional configuration, said bar includes a projection partially overlying said slot and running the length of said slot, said locking strip is spoon shaped in cross sectional configuration including a base portion of similar half moon cross sectional configuration to said slot and having a lip underlying said clamping bar projection, and said locking strip further includes a handle like projection in engagement with said channel member to prevent rotational release of the locking strip from the slot during lateral shifting of the bar members relative to the longitudinAl axis of said channel members.
 8. The adjustable frame assembly as claimed in claim 7, wherein the channel wall remote from said screen is recessed longitudinally and the locking strip handle like projection is received by said recess.
 9. The adjustable frame assembly as claimed in claim 8, wherein said recess within said channel wall is inclined, the handle like projection of said locking strip carries an inclined edge which mates with the inclined surface of said recess when said bar is in said second position and said locking strip is prevented from rotating in a first direction by said lip contacting said bar member projection and said locking strip projection contacting one wall of said channel wall recess, and said locking strip is prevented from rotating in the opposite direction by said locking strip projection, contacting the other wall of said channel wall recess. 